This invention relates to limited bandwidth mixers having a performance approximating that of a square-wave reversing mixer.
Mixers convert a frequency of an incoming signal to an intermediate frequency, consisting of a difference between the signal frequency and a preset pump frequency in the mixer. A high performance mixer for ultra-high frequency (UHF) and microwave signals is the Paramixer.TM., described in U.S. Pat. Nos. 4,112,374 and 4,230,956, and available as a component in digital receivers from Steinbrecher Corporation of Burlington, Mass. The Paramixer has a balanced and symmetrical configuration containing eight diodes that closely approximates the performance of an ideal square-wave reversing switch mixer over a broad band of input signal frequencies, ranging from 10 to 1000 MHz. The fundamental limit on conversion loss for the Paramixer is 3.92 dB when resistive terminations are applied to all frequencies, and 0 dB when all frequencies except the intermediate frequency and signal frequency are terminated properly with lossless circuit elements. A square wave local oscillator signal in the Paramixer switches the diodes rapidly between well-defined ON and OFF states to provide the Paramixer's high intercept or high dynamic range.
In theory, a single diode ideal H-mixer 10, shown in FIG. 1, also may achieve the performance of a square wave reversing switch mixer when driven by a sinusoidal pump wave under certain boundary conditions. (A. A. M. Saleh, "Theory of Resistive Mixers", Ph.D. Thesis, Massachusetts Institute of Technology, January 1970). The boundary conditions include supplying the pump wave at an appropriate drive level, and maintaining an intermediate frequency much lower than the pump frequency. Other boundary conditions limit the mixer to exchanging power only at the signal frequency, the intermediate frequency and an image frequency, consisting of the difference between the pump frequency and the intermediate frequency.
The H-mixer 10 receives both the pump wave (V.sub.p) and the signal (V.sub.s) at an input port 12. An ideal input filter 14 consisting of an inductor 16 and capacitor 18 in series allows a sinusoidal current to exist at only the signal frequency (.omega..sub.s), the pump frequency (.omega..sub.p) and the image frequency (.omega..sub.i =2.omega..sub.p -.omega..sub.s).
A parallel transmission line stub 20 is connected between the input filter and an exponential diode 22. The transmission line stub has a length equal to one quarter the wavelength of oscillation of the pump wave (.lambda..sub.p =2.pi./.omega..sub.p), and thus presents a short circuit to even harmonics of the pump wave and presents an open circuit to odd harmonics of the pump wave. As a result, the voltage across the diode contains only odd harmonics of the pump frequency, and the current through the diode contains only even harmonics of the pump frequency in addition to the fundamental current.
Diode bias current passes through a grounded LC tank 24 consisting of a parallel capacitor 26 and diode 28 connected to an output port 30 of the mixer 10. The LC tank supports a voltage at the intermediate frequency (.omega..sub.0 =.omega..sub.s -.omega..sub.p) only, while short-circuiting voltages at all other frequencies. The voltage across the output port of the mixer thus oscillates at the intermediate frequency, as required.
The mixer 10 also has a battery 32 connected in series with the LC tank that establishes DC bias for the diode. A capacitor 34 connected in parallel to the battery provides a bypass for all AC currents in the circuit.
Another ideal switch mixer, seen in FIG. 2, is an ideal G-mixer 60, a dual of the H-mixer described above. The G-mixer receives the signal and pump wave at an input port 62. A grounded LC tank 64 consisting of a parallel capacitor 66 and an inductor 68 is connected between the input port of the mixer and an exponential diode 70. The LC tank is an ideal filter supporting a voltage at only the signal frequency, the image frequency and the pump frequency. A battery 72 connected to a choke 74 in series with the diode and the LC tank provides the bias for the diode. AC currents flow between the diode and the LC tank through a quarter pump wavelength transmission line stub 76 that presents a short circuit to odd harmonics and an open circuit to even harmonics of the pump frequency. An ideal series LC filter 78 between the diode and an output port 80 of the mixer 60 ensures that only currents at the intermediate frequency are present at the output of the mixer.